Chemical-free production of graphene materials

ABSTRACT

A method of producing isolated graphene sheets directly from a graphitic material, comprising: a) mixing multiple particles of a graphitic material and multiple particles of a solid carrier material to form a mixture in an impacting chamber of an energy impacting apparatus; b) operating the impacting apparatus for peeling off graphene sheets from the graphitic material and transferring these graphene sheets to surfaces of solid carrier material particles to produce graphene-coated solid particles inside the impacting chamber; c) separating the graphene sheets from the solid carrier material particle surfaces to recover isolated graphene sheets. The method enables production of graphene sheets directly from a graphitic material without going through a chemical intercalation or oxidation procedure. The process is fast (hours as opposed to days of conventional processes), has low or no water usage, environmentally benign, cost effective, and highly scalable.

FIELD OF THE INVENTION

The present invention relates to the art of graphene materials and, inparticular, to a method of producing isolated graphene sheets in anenvironmentally benign manner.

BACKGROUND

A single-layer graphene sheet is composed of carbon atoms occupying atwo-dimensional hexagonal lattice. Multi-layer graphene is a plateletcomposed of more than one graphene plane. Individual single-layergraphene sheets and multi-layer graphene platelets are hereincollectively called nano graphene platelets (NGPs) or graphenematerials. NGPs include pristine graphene (essentially 99% of carbonatoms), slightly oxidized graphene (<5% by weight of oxygen), grapheneoxide (≥5% by weight of oxygen), slightly fluorinated graphene (<5% byweight of fluorine), graphene fluoride ((≥5% by weight of fluorine),other halogenated graphene, and chemically functionalized graphene.

NGPs have been found to have a range of unusual physical, chemical, andmechanical properties. For instance, graphene was found to exhibit thehighest intrinsic strength and highest thermal conductivity of allexisting materials. Although practical electronic device applicationsfor graphene (e.g., replacing Si as a backbone in a transistor) are notenvisioned to occur within the next 5-10 years, its application as anano filler in a composite material and an electrode material in energystorage devices is imminent. The availability of processable graphenesheets in large quantities is essential to the success in exploitingcomposite, energy, and other applications for graphene.

Our research group was among the first to discover graphene [B. Z. Jangand W. C. Huang, “Nano-scaled Graphene Plates,” U.S. patent applicationSer. No. 10/274,473, submitted on Oct. 21, 2002; now U.S. Pat. No.7,071,258 (Jul. 4, 2006)]. The processes for producing NGPs and NGPnanocomposites were recently reviewed by us [Bor Z. Jang and A Zhamu,“Processing of Nano Graphene Platelets (NGPs) and NGP Nanocomposites: AReview,” J. Materials Sci. 43 (2008) 5092-5101]. Our research hasyielded a process for chemical-free production of isolated nano grapheneplatelets that is novel in that is does not follow the establishedmethods for production of nano graphene platelets outlined below. Inaddition, the process is of enhanced utility in that it is costeffective, and provided novel graphene materials with significantlyreduced environmental impact. Four main prior-art approaches have beenfollowed to produce NGPs. Their advantages and shortcomings are brieflysummarized as follows:

Approach 1: Chemical Formation and Reduction of Graphite Oxide (GO)Platelets

The first approach (FIG. 1 ) entails treating natural graphite powderwith an intercalant and an oxidant (e.g., concentrated sulfuric acid andnitric acid, respectively) to obtain a graphite intercalation compound(GIC) or, actually, graphite oxide (GO). [William S. Hummers, Jr., etal., Preparation of Graphitic Oxide, Journal of the American ChemicalSociety, 1958, p. 1339.] Prior to intercalation or oxidation, graphitehas an inter-graphene plane spacing of approximately 0.335 nm(L_(d)=½d₀₀₂=0.335 nm). With an intercalation and oxidation treatment,the inter-graphene spacing is increased to a value typically greaterthan 0.6 nm. This is the first expansion stage experienced by thegraphite material during this chemical route. The obtained GIC or GO isthen subjected to further expansion (often referred to as exfoliation)using either a thermal shock exposure or a solution-based,ultrasonication-assisted graphene layer exfoliation approach.

In the thermal shock exposure approach, the GIC or GO is exposed to ahigh temperature (typically 800-1,050° C.) for a short period of time(typically 15 to 60 seconds) to exfoliate or expand the GIC or GO forthe formation of exfoliated or further expanded graphite, which istypically in the form of a “graphite worm” composed of graphite flakesthat are still interconnected with one another. This thermal shockprocedure can produce some separated graphite flakes or graphene sheets,but normally the majority of graphite flakes remain interconnected.Typically, the exfoliated graphite or graphite worm is then subjected toa flake separation treatment using air milling, mechanical shearing, orultrasonication in water. Hence, approach 1 basically entails threedistinct procedures: first expansion (oxidation or intercalation),further expansion (or “exfoliation”), and separation.

In the solution-based separation approach, the expanded or exfoliated GOpowder is dispersed in water or aqueous alcohol solution, which issubjected to ultrasonication. It is important to note that in theseprocesses, ultrasonification is used after intercalation and oxidationof graphite (i.e., after first expansion) and typically after thermalshock exposure of the resulting GIC or GO (after second expansion).Alternatively, the GO powder dispersed in water is subjected to an ionexchange or lengthy purification procedure in such a manner that therepulsive forces between ions residing in the inter-planar spacesovercome the inter-graphene van der Waals forces, resulting in graphenelayer separations.

There are several major problems associated with this conventionalchemical production process:

-   -   (1) The process requires the use of large quantities of several        undesirable chemicals, such as sulfuric acid, nitric acid, and        potassium permanganate or sodium chlorate.    -   (2) The chemical treatment process requires a long intercalation        and oxidation time, typically 5 hours to five days.    -   (3) Strong acids consume a significant amount of graphite during        this long intercalation or oxidation process by “eating their        way into the graphite” (converting graphite into carbon dioxide,        which is lost in the process). It is not unusual to lose 20-50%        by weight of the graphite material immersed in strong acids and        oxidizers.    -   (4) The thermal exfoliation requires a high temperature        (typically 800-1,200° C.) and, hence, is a highly        energy-intensive process.    -   (5) Both heat- and solution-induced exfoliation approaches        require a very tedious washing and purification step. For        instance, typically 2.5 kg of water is used to wash and recover        1 gram of GIC, producing huge quantities of waste water that        need to be properly treated.    -   (6) In both the heat- and solution-induced exfoliation        approaches, the resulting products are GO platelets that must        undergo a further chemical reduction treatment to reduce the        oxygen content. Typically even after reduction, the electrical        conductivity of GO platelets remains much lower than that of        pristine graphene. Furthermore, the reduction procedure often        involves the utilization of toxic chemicals, such as hydrazine.    -   (7) Furthermore, the quantity of intercalation solution retained        on the flakes after draining may range from 20 to 150 parts of        solution by weight per 100 parts by weight of graphite flakes        (pph) and more typically about 50 to 120 pph. During the        high-temperature exfoliation, the residual intercalate species        retained by the flakes decompose to produce various species of        sulfuric and nitrous compounds (e.g., NO_(x) and SO_(x)), which        are undesirable. The effluents require expensive remediation        procedures in order not to have an adverse environmental impact.        The present invention was made to overcome the limitations        outlined above.        Approach 2: Direct Formation of Pristine Nano Graphene Platelets

In 2002, our research team succeeded in isolating single-layer andmulti-layer graphene sheets from partially carbonized or graphitizedpolymeric carbons, which were obtained from a polymer or pitch precursor[B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/274,473, submitted on Oct. 21, 2002; now U.S.Pat. No. 7,071,258 (Jul. 4, 2006)]. Mack, et al [“Chemical manufactureof nanostructured materials” U.S. Pat. No. 6,872,330 (Mar. 29, 2005)]developed a process that involved intercalating graphite with potassiummelt and contacting the resulting K-intercalated graphite with alcohol,producing violently exfoliated graphite containing NGPs. The processmust be carefully conducted in a vacuum or an extremely dry glove boxenvironment since pure alkali metals, such as potassium and sodium, areextremely sensitive to moisture and pose an explosion danger. Thisprocess is not amenable to the mass production of NGPs. The presentinvention was made to overcome the limitations outlined above.

Approach 3: Epitaxial Growth and Chemical Vapor Deposition of NanoGraphene Sheets on Inorganic Crystal Surfaces

Small-scale production of ultra-thin graphene sheets on a substrate canbe obtained by thermal decomposition-based epitaxial growth and a laserdesorption-ionization technique. [Walt A. DeHeer, Claire Berger, PhillipN. First, “Patterned thin film graphite devices and method for makingsame” U.S. Pat. No. 7,327,000 B2 (Jun. 12, 2003)] Epitaxial films ofgraphite with only one or a few atomic layers are of technological andscientific significance due to their peculiar characteristics and greatpotential as a device substrate. However, these processes are notsuitable for mass production of isolated graphene sheets for compositematerials and energy storage applications. The present invention wasmade to overcome the limitations outlined above.

Approach 4: The Bottom-Up Approach (Synthesis of Graphene from SmallMolecules)

Yang, et al. [“Two-dimensional Graphene Nano-ribbons,” J. Am. Chem. Soc.130 (2008) 4216-17] synthesized nano graphene sheets with lengths of upto 12 nm using a method that began with Suzuki-Miyaura coupling of1,4-diiodo-2,3,5,6-tetraphenyl-benzene with 4-bromophenylboronic acid.The resulting hexaphenylbenzene derivative was further derivatized andring-fused into small graphene sheets. This is a slow process that thusfar has produced very small graphene sheets. The present invention wasmade to overcome the limitations outlined above.

Hence, an urgent need exists to have a graphene production process thatrequires a reduced amount of undesirable chemical (or elimination ofthese chemicals all together), shortened process time, less energyconsumption, lower degree of graphene oxidation, reduced or eliminatedeffluents of undesirable chemical species into the drainage (e.g.,sulfuric acid) or into the air (e.g., SO₂ and NO₂). The process shouldbe able to produce more pristine (less oxidized and damaged), moreelectrically conductive, and larger/wider graphene sheets.

SUMMARY OF THE INVENTION

The present invention provides a strikingly simple, fast, scalable,environmentally benign, and cost-effective process that meets theaforementioned needs. This method of producing single-layer or few layergraphene directly from a graphitic or carbonaceous material (a graphenesource material) comprises subjecting a mixture of graphitic material,particles of a solid carrier material, and, optionally, impacting ballsto mechanical agitation via a ball mill or similar energy impactingdevice for a length of time sufficient for peeling off graphene layers(planes of hexagonally arranged carbon atoms) from the source graphitematerial, and coating these peeled-off graphene layers onto surfaces ofthe solid carrier material particles. With the presence of impactingballs, graphene sheets can be peeled off from the source graphiteparticles and tentatively deposited onto the surfaces of impactingballs. When these graphene sheet-coated impacting balls subsequentlyimpinge upon solid carrier particles, the graphene sheets aretransferred to surfaces of carrier particles. The solid carrier materialis then removed (separated from the graphene sheets) by dissolving,burning, sublimation, melting or other process, leaving behind isolatedgraphene sheets, also referred to as nano graphene platelets (NGP).

Preferably, the starting material (graphitic or carbonaceous material asa graphene source material) has never been previously intercalated orchemically oxidized. This starting material is not a graphiteintercalation compound (GIC) or graphite oxide (GO). Preferably, thesource graphitic material may be selected from natural graphite,synthetic graphite, highly oriented pyrolytic graphite, meso-carbonmicro-bead, graphite fiber, graphitic nano-fiber, graphite oxide,graphite fluoride, chemically modified graphite, exfoliated graphite,vein graphite, or a combination thereof.

In some embodiments, the impacting chamber of the energy impactingapparatus further contains a protective fluid; e.g. inert gas,non-reactive liquid, water, etc.

This is essentially a two-step process, significantly reducing processcosts. In less than 4 hours of process time, graphene sheets are peeledoff from graphite particles, followed by a fast, efficient removal ofthe carrier material. This process is stunningly short and simple.

A preferred embodiment of the present invention is a method of directlymixing a graphitic material and a carrier material into an energyimpacting device, such as a ball mill, and submitting the mixture to asufficiently long treatment time to peel off graphene layers from thesource graphitic material and transfer these graphene layers immediatelyto the carrier material surfaces. These graphene layers are typicallysingle-layer or few-layer graphene sheets (typically <5 layers; mostlysingle-layer graphene). Following this step, the carrier layer isremoved by dissolution, burning, sublimation, melting or other method.The end result is isolated graphene sheets.

This is quite surprising, considering prior researchers andmanufacturers have focused on more complex, time intensive and costlymethods to create graphene in industrial quantities. In other words, ithas been believed that chemical intercalation and oxidation is needed toproduce bulk quantities of graphene platelets. The present inventiondefies this expectation in many ways:

-   -   (1) Unlike the chemical intercalation and oxidation (which        requires expansion of inter-graphene spaces, further expansion        or exfoliation of graphene planes, and full separation of        exfoliated graphene sheets), the instant method directly removes        graphene sheets from a source graphitic material and transfers        these graphene sheets to surfaces of carrier material particles.        No undesirable chemicals (e.g. sulfuric acid and nitric acid)        are used.    -   (2) Unlike oxidation and intercalation, pristine graphene sheets        can be transferred onto the carrier material. The sheets being        free of oxidation damage allow the creation of graphene        containing products with higher electrical and thermal        conductivity.    -   (3) Unlike bottom up production methods, large continuous        platelets can be produced with the instant method.    -   (4) Contrary to common production methods, strong acids and        oxidizers are not needed to create the graphene coating.    -   (5) Contrary to common production methods, a washing process        requiring substantial amounts of water is not needed.

The carrier material can be an organic, inorganic, metal, glass, metaloxide, metal carbide, metal nitride, metal boride, or ceramic material.Carrier materials can be in the form of pellets, filament, fibers,powder, reactor spheres, or other forms. Some examples of organiccarrier materials include wax, soluble polymers, nut husks, and woodchips.

The energy impacting apparatus may be selected from a ball mill,vibratory ball mill, planetary ball mill, high energy mill, basket mill,agitator ball mill, continuous ball mill, stirred ball mill, pressurizedball mill, vacuum ball mill, freezer (SPEX) mill, vibratory sieve,ultrasonic homogenizer mill, resonant acoustic mixer, or shaker table.

The presently invented process is capable of producing single-layergraphene sheets. In many examples, the graphene material producedcontains at least 80% single-layer graphene sheets. The grapheneproduced can contain pristine graphene, oxidized graphene with less than5% oxygen content by weight, graphene fluoride with less than 5%fluorine by weight, graphene with a carbon content no less than 95% byweight, or functionalized graphene.

Another surprising and highly advantageous feature of the presentlyinvented process is the notion that graphene sheet production andchemical functionalization can be accomplished concurrently in the sameimpacting chamber. The impact-induced kinetic energy experienced by thecarrier particles are of sufficient energy and intensity to chemicallyactivate the edges and surfaces of graphene sheets coated on carrierparticle surfaces; e.g. creating highly active sites or free radicals).Desired functional groups can be imparted to graphene edges and/orsurfaces, provided that selected chemical species (functionalizingagents) containing desired chemical function groups (e.g. —NH₂, Br—,etc.) are dispersed in the impacting chamber. Chemical functionalizationreactions can occur in situ as soon as the reactive sites or activeradicals are formed.

In some embodiments, functionalizing agents contain a chemicalfunctional group selected from functional group is selected from alkylor aryl silane, alkyl or aralkyl group, hydroxyl group, carboxyl group,amine group, sulfonate group (—SO₃H), aldehydic group, quinoidal,fluorocarbon, or a combination thereof.

Alternatively, the functionalizing agent contains an azide compoundselected from the group consisting of 2-Azidoethanol,3-Azidopropan-1-amine, 4-(2-Azidoethoxy)-4-oxobutanoic acid,2-Azidoethyl-2-bromo-2-methylpropanoate, chlorocarbonate,azidocarbonate, dichlorocarbene, carbene, aryne, nitrene,(R-)-oxycarbonyl nitrenes, where R=any one of the following groups,

and combinations thereof.

In certain embodiments, the functionalizing agent contains an oxygenatedgroup selected from the group consisting of hydroxyl, peroxide, ether,keto, and aldehyde. In certain embodiments, the functionalizing agentcontains a functional group selected from the group consisting of SO₃H,COOH, NH₂, OH, R′CHOH, CHO, CN, COCl, halide, COSH, SH, COOR′, SR′,SiR′₃, Si(—OR′—)_(y)R′₃-y, Si(—O—SiR′₂—)OR′, R″, Li, AlR′₂, Hg—X, TlZ₂and Mg—X; wherein y is an integer equal to or less than 3, R′ ishydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, orpoly(alkylether), R″ is fluoroalkyl, fluoroaryl, fluorocycloalkyl,fluoroaralkyl or cycloaryl, X is halide, and Z is carboxylate ortrifluoroacetate, and combinations thereof.

The functionalizing agent may contain a functional group is selectedfrom the group consisting of amidoamines, polyamides, aliphatic amines,modified aliphatic amines, cycloaliphatic amines, aromatic amines,anhydrides, ketimines, diethylenetriamine (DETA), triethylene-tetramine(TETA), tetraethylene-pentamine (TEPA), polyethylene polyamine,polyamine epoxy adduct, phenolic hardener, non-brominated curing agent,non-amine curatives, and combinations thereof.

In some embodiments, the functionalizing agent contains a functionalgroup selected from OY, NHY, O═C—OY, P═C—NR′Y, O═C—SY, O═C—Y, —CR′1-OY,NY or C′Y, and Y is a functional group of a protein, a peptide, an aminoacid, an enzyme, an antibody, a nucleotide, an oligonucleotide, anantigen, or an enzyme substrate, enzyme inhibitor or the transitionstate analog of an enzyme substrate or is selected from R′—OH, R′—NR′₂,R′SH, R′CHO, R′CN, R′X, R′N⁺(R′)₃X⁻, R′SiR′₃, R′Si(—OR′—)_(y)R′_(3-y),R′—R″, R′—N—CO, (C₂H₄O—)_(w)H, (—C₃H₆)—)_(w)H, (—C₂H₄O)_(w)—R′,(C₃H₆O)_(w)—R′, R′, and w is an integer greater than one and less than200.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A flow chart showing the most commonly used prior art process ofproducing highly oxidized NGPs that entails tedious chemicaloxidation/intercalation, rinsing, and high-temperature exfoliationprocedures.

FIG. 2 A flow chart showing the presently invented two-step process forproducing isolated graphitic materials

FIG. 3 A flow chart showing the presently invented process for producingisolated graphitic materials via a continuous ball mill.

FIG. 4(A) Transmission electron micrograph of graphene sheets producedby conventional Hummer's route (much smaller graphene sheets, butcomparable thickness).

FIG. 4(B) Transmission electron micrograph of graphene sheets producedby the presently invented impact energy method.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Carbon materials can assume an essentially amorphous structure (glassycarbon), a highly organized crystal (graphite), or a whole range ofintermediate structures that are characterized in that variousproportions and sizes of graphite crystallites and defects are dispersedin an amorphous matrix. Typically, a graphite crystallite is composed ofa number of graphene sheets or basal planes that are bonded togetherthrough van der Waals forces in the c-axis direction, the directionperpendicular to the basal plane. These graphite crystallites aretypically micron- or nanometer-sized. The graphite crystallites aredispersed in or connected by crystal defects or an amorphous phase in agraphite particle, which can be a graphite flake, carbon/graphite fibersegment, carbon/graphite whisker, or carbon/graphite nano-fiber.

One preferred specific embodiment of the present invention is a methodof producing a nano graphene platelet (NGP) material that is essentiallycomposed of a sheet of graphene plane (hexagonal lattice of carbonatoms) or multiple sheets of graphene plane stacked and bonded together(typically, on an average, less than five sheets per multi-layerplatelet). Each graphene plane, also referred to as a graphene sheet orbasal plane comprises a two-dimensional hexagonal structure of carbonatoms. Each platelet has a length and a width parallel to the graphiteplane and a thickness orthogonal to the graphite plane. By definition,the thickness of an NGP is 100 nanometers (nm) or smaller, with asingle-sheet NGP being as thin as 0.34 nm. However, the NGPs producedwith the instant methods are mostly single-layer graphene with somefew-layer graphene sheets (<5 layers). The length and width of a NGP aretypically between 200 nm and 20 μm, but could be longer or shorter,depending upon the sizes of source graphite material particles.

The present invention provides a strikingly simple, fast, scalable,environmentally benign, and cost-effective process that avoidsessentially all of the drawbacks associated with prior art processes. Asschematically illustrated in FIG. 2 , one preferred embodiment of thismethod entails placing source graphitic material particles and carriermaterial particles (plus optional impacting balls, if so desired) in animpacting chamber. After loading, the resulting mixture is immediatelyexposed to impacting energy, which is accomplished by rotating thechamber to enable the impacting of the carrier particles (and optionalimpacting balls) against graphite particles. These repeated impactingevents (occurring in high frequencies and high intensity) act to peeloff graphene sheets from the surfaces of graphitic material particlesand directly transfer these graphene sheets to the surfaces of carrierparticles. This is a “direct transfer” process.

Alternatively, in the impacting chambers containing impacting balls(e.g. stainless steel or zirconia beads), graphene sheets are peeled offby the impacting balls and tentatively transferred to the surfaces ofimpacting balls first. When the graphene-coated impacting balls impingeupon the carrier material particles, the graphene sheets are transferredto surfaces of the carrier material particles. This is an “indirecttransfer” process.

In less than four hours, most of the constituent graphene sheets ofsource graphite particles are peeled off, forming mostly single-layergraphene and few-layer graphene (mostly less than 5 layers). Followingthe direct or indirect transfer process (coating of graphene sheets oncarrier material particles), the graphene sheets can be separated fromthe carrier material particles using a broad array of methods. Forinstance, the carrier material (e.g. plastic or organic material) isignited, burning away the carrier material and leaving behind isolatednano graphene platelets. The carrier material (e.g. wax, polymer, orsalt) may be dissolved in a benign solvent (e.g. water, if the carrieris a water soluble material). There are many water soluble polymers(e.g. polyacrylamide and polyvinyl alcohol) and many water soluble salts(e.g. NaCl) that can be used for this purpose. If necessary, organicsolvents can be used to dissolve most of the thermoplastic materials.The graphene sheets coated on metal, glass, or ceramic particles (metaloxide, metal carbide, etc.) can be easily detached from the particles byimmersing the coated particles in a water bath and subjected the slurryto mild ultrasonication, for instance. This graphene production processis stunningly short and simple, and highly scalable.

In contrast, as shown in FIG. 1 , the prior art chemical processestypically involve immersing graphite powder in a mixture of concentratedsulfuric acid, nitric acid, and an oxidizer, such as potassiumpermanganate or sodium perchlorate, forming a reacting mass thatrequires typically 5-120 hours to complete the chemicalintercalation/oxidation reaction. Once the reaction is completed, theslurry is subjected to repeated steps of rinsing and washing with waterand then subjected to drying treatments to remove water. The driedpowder, referred to as graphite intercalation compound (GIC) or graphiteoxide (GO), is then subjected to a thermal shock treatment. This can beaccomplished by placing GIC in a furnace pre-set at a temperature oftypically 800-1100° C. (more typically 950-1050° C.). The resultingproducts are typically highly oxidized graphene (i.e. graphene oxidewith a high oxygen content), which must be chemically or thermal reducedto obtain reduced graphene oxide (RGO). RGO is found to contain a highdefect population and, hence, is not as conducting as pristine graphene.We have observed that that the pristine graphene paper (prepared byvacuum-assisted filtration of pristine graphene sheets) exhibitelectrical conductivity values in the range of 1,500-4,500 S/cm. Incontrast, the RGO paper prepared by the same paper-making proceduretypically exhibits electrical conductivity values in the range of100-1,000 S/cm.

It is again critically important to recognize that the impacting processnot only avoids significant chemical usage, but also produces a higherquality final product—pristine graphene as opposes to thermally reducedgraphene oxide, as produced by the prior art process. Pristine grapheneenables the creation of materials with higher electrical and thermalconductivity.

Although the mechanisms remain incompletely understood, thisrevolutionary process of the present invention appears to essentiallyeliminate the required functions of graphene plane expansion,intercalant penetration, exfoliation, and separation of graphene sheetsand replace it with an entirely mechanical exfoliation process. Thewhole process can take less than 5 hours, and can be done with no addedchemicals. This is absolutely stunning, a shocking surprise to eventhose top scientists and engineers or those of extraordinary ability inthe art.

Another surprising result of the present study is the observation that awide variety of carbonaceous and graphitic materials can be directlyprocessed without any particle size reduction or pre-treatment. Thismaterial may be selected from natural graphite, synthetic graphite,highly oriented pyrolytic graphite, meso-carbon micro-bead, graphitefiber, graphitic nano-fiber, graphite oxide, graphite fluoride,chemically modified graphite, exfoliated graphite, or a combinationthereof. By contrast, graphitic material for used for the prior artchemical formation and reduction of graphene oxide requires sizereduction to 75 um or less average particle size. This process requiressize reduction equipment (for example hammer mills or screening mills),energy input, and dust mitigation. By contrast, the energy impactingdevice method can accept almost any size of graphitic material. Startingmaterial of mm or cm in size or larger has been successfully processedto create graphene. The only size limitation is the chamber capacity ofthe energy impacting device.

The presently invented process is capable of producing single-layergraphene sheets. In many examples, the graphene material producedcontains at least 80% single-layer graphene sheets. The grapheneproduced can contain pristine graphene, oxidized graphene with less than5% oxygen content by weight, graphene fluoride, graphene oxide with lessthan 5% fluorine by weight, graphene with a carbon content no less than95% by weight, or functionalized graphene.

The presently invented process does not involve the production of GICand, hence, does not require the exfoliation of GIC at a highexfoliation temperature (e.g. 800-1,100° C.). This is another majoradvantage from environmental protection perspective. The prior artprocesses require the preparation of dried GICs containing sulfuric acidand nitric acid intentionally implemented in the inter-graphene spacesand, hence, necessarily involve the decomposition of H₂SO₄ and HNO₃ toproduce volatile gases (e.g. NO_(x) and SO_(x)) that are highlyregulated environmental hazards. The presently invented processcompletely obviates the need to decompose H₂SO₄ and HNO₃ and, hence, isenvironmentally benign. No undesirable gases are released into theatmosphere during the combined graphite expansion/exfoliation/separationprocess of the present invention.

One preferred embodiment of the present invention is the inclusion ofimpacting balls or media to the impacting chamber, as illustrated inFIG. 2 . The impact media may contain balls of metal, glass, ceramic, ororganic materials. The size of the impacting media may be in the rangeof 1 mm to 20 mm, or it may be larger or smaller. The shape of theimpacting media may be spherical, needle like, cylindrical, conical,pyramidal, rectilinear, or other shapes. A combination of shapes andsizes may be selected. The size distribution may be unimodal Gaussian,bimodal or tri-modal.

Another preferred embodiment of this method is removing the carriermaterial through burning, leaving behind isolated nano grapheneplatelets. Unlike the high energy input required for the prior artprocess to reach 1050° C. to thermally reduce graphene oxide, burn outof organic materials is an exothermic process. Once initiated in acontinuous oven or furnace, the energy required to maintain burn outtemperature is modest and does not impact the cost effectiveness of theprocess.

Another preferred embodiment of this method is removing the carriermaterial through a mixture of burn out (oxygen atmosphere) and reduction(inert or reducing atmosphere). In this embodiment, the carrier materialis ignited at room temperature or in an oven. The oven temperature ispreferably 30° C. to 1000° C., more preferably 200° C.-800° C., and morepreferably 350° C.-450° C. The burn out process removes 90 to 99.9% ofthe carrier material. In this embodiment, a second thermal process iscarried out at greater than 600° C. in an inert or reducing atmosphereto remove remaining carrier material.

Another preferred embodiment of this method is removing the carriermaterial through melting, leaving behind isolated nano grapheneplatelets. This process has allows for re-use of the carrier material,reducing process cost and environmental impact. Unlike the hightemperatures required for the prior art process of thermal reduction ofgraphene oxide to produce graphene, the temperatures required for amelting process are comparatively low: for example 46° C. to 200° C. forwax, 231.9° C. for tin, 166° C. for poly lactic acid polymer. The energyinput required to remove the carrier from graphene is modest and doesnot impact the cost effectiveness of the process.

Another preferred embodiment of this method is removing the carriermaterial through dissolution. For example,acrylonitrile-butadiene-styrene (ABS) carrier material can be dissolvedin acetone. After washing over a filter to remove residual ABS, thefiltrate can be vacuum dried, spray dried, or air dried, resulting inisolated graphene material. The solvents used in this process can berecovered via solvent recovery processes including vacuum distillation,this is a cost effective process.

Another preferred embodiment of this method is removing the carriermaterial through sublimation. The carrier material may be selected fromice or iodine crystals. If ice is used as the carrier material, acommercially available freeze dryer can be used to remove the carriervia sublimation. A major benefit of the sublimation approach is theavoidance of the exposure of NGP to surface tension, which can causemorphology changes or agglomeration. Another benefit of sublimationprocess is the ability to choose a carrier material that will leave nochemical residue on the graphene material.

One significant advantage of the present invention as compared to priorart is flexibility of selecting carrier materials. There are manyopportunities to use pre-consumer, or post-consumer waste material asthe carrier, diverting this material from disposal by landfill orincineration. Nut shells, rice husks, shredded tires, and groundco-mingled recycled plastic are all possible cost effective carriermaterials for the production of graphene. An additional advantage ofthis flexibility is the carrier material can be chosen to maximizeburnout or to maximize solubility in low cost, environmentally friendly,and lower health hazard hazardous solvents. For example, a carriermaterial readily soluble in acetone may be chosen over a carriermaterial requiring methyl ethyl ketone for removal, and will facilitatethe scale up of a cost effective manufacturing process.

An important advantage of the present invention is the opportunity touse multiple carrier removal methods. For example, melting at 200° maybe used to remove 99% of a wax carrier material, followed by burnout at1000° C. As another example, solvent removal may be used to remove themajority of a polymer carrier material, followed by burnout at 1200° C.The combining of carrier removal processes allows for cost effectivere-use of carrier materials, while also achieving high levels of purityfor the final graphene produced.

In a desired embodiment, the presently invented method is carried out inan automated and/or continuous manner. For instance, as illustrated inFIG. 3 , the mixture of graphite particles and solid carrier particles(plus optional impacting balls) is delivered by a conveyer belt 12 andfed into a continuous ball mill 14. After ball milling to formgraphene-coated solid carrier particles, the product mixture (possiblyalso containing some residual graphite particles and optional impactingballs) is discharged from the ball mill apparatus 14 into a screeningdevice (e.g. a rotary drum 16) to separate graphene-coated solid carrierparticles from residual graphite particles (if any) and impacting balls(if any). This separation operation may be assisted by a, magneticseparator 18 if the impacting balls are ferromagnetic (e.g. balls of Fe,Co, Ni, or Mn-based metal). The graphene-coated carrier particles may bedelivered into a combustion chamber 20, if the solid carrier can beburned off (e.g. plastic beads, rubber particles, and wax particles,etc.). Alternatively, these particles can be discharged into adissolving chamber for dissolving the carrier particles (e.g. plasticbeads). The product mass can be further screened in another (optional)screening device 22, a powder classifier or cyclone 24, and/or anelectrostatic separator 26. These procedures can be fully automated.

Preferred Mode of Chemical Functionalization

Graphene sheets transferred to carrier solid particle surfaces have asignificant proportion of surfaces that correspond to the edge planes ofgraphite crystals. The carbon atoms at the edge planes are reactive andmust contain some heteroatom or group to satisfy carbon valency. Thereare many types of functional groups (e.g. hydroxyl and carboxylic) thatare naturally present at the edge or surface of graphene nano plateletsproduced through transfer to a solid carrier particle. Theimpact-induced kinetic energy experienced by the carrier particles areof sufficient energy and intensity to chemically activate the edges andeven surfaces of graphene sheets coated on carrier particle surfaces(e.g. creating highly active sites or free radicals). Provided thatcertain chemical species containing desired chemical function groups(e.g. —NH₂, Br—, etc.) are included in the impacting chamber, thesefunctional groups can be imparted to graphene edges and/or surfaces. Inother words, production and chemical functionalization of graphenesheets can be accomplished concurrently by including appropriatechemical compounds in the impacting chamber. In summary, a majoradvantage of the present invention over other processes is thesimplicity of simultaneous production and modification of surfacechemistry.

Graphene platelets derived by this process may be functionalized throughthe inclusion of various chemical species in the impacting chamber. Ineach group of chemical species discussed below, we selected 2 or 3chemical species for functionalization studies.

In one preferred group of chemical agents, the resulting functionalizedNGP may broadly have the following formula(e): [NGP]-R_(m), wherein m isthe number of different functional group types (typically between 1 and5), R is selected from SO₃H, COOH, NH₂, OH, R′CHOH, CHO, CN, COCl,halide, COSH, SH, COOR′, SR′, SiR′₃, R″, Li, AlR′₂, Hg—X, TlZ₂ and Mg—X;wherein y is an integer equal to or less than 3, R′ is hydrogen, alkyl,aryl, cycloalkyl, or aralkyl, cycloaryl, or poly(alkylether), R″ isfluoroalkyl, fluoroaryl, fluorocycloalkyl, fluoroaralkyl or cycloaryl, Xis halide, and Z is carboxylate or trifluoroacetate.

For NGPs to be effective reinforcement fillers in epoxy resin, thefunction group —NH₂ is of particular interest. For example, a commonlyused curing agent for epoxy resin is diethylenetriamine (DETA), whichhas three —NH₂ groups. If DETA is included in the impacting chamber, oneof the three —NH₂ groups may be bonded to the edge or surface of agraphene sheet and the remaining two un-reacted —NH₂ groups will beavailable for reacting with epoxy resin later. Such an arrangementprovides a good interfacial bonding between the NGP (graphene sheets)and the matrix resin of a composite material.

Other useful chemical functional groups or reactive molecules may beselected from the group consisting of amidoamines, polyamides, aliphaticamines, modified aliphatic amines, cycloaliphatic amines, aromaticamines, anhydrides, ketimines, diethylenetriamine (DETA),triethylene-tetramine (TETA), tetraethylene-pentamine (TEPA),hexamethylenetetramine, polyethylene polyamine, polyamine epoxy adduct,phenolic hardener, non-brominated curing agent, non-amine curatives, andcombinations thereof. These functional groups are multi-functional, withthe capability of reacting with at least two chemical species from atleast two ends. Most importantly, they are capable of bonding to theedge or surface of graphene using one of their ends and, duringsubsequent epoxy curing stage, are able to react with epoxide or epoxyresin at one or two other ends.

The above-described [NGP]-R_(m) may be further functionalized. This canbe conducted by opening up the lid of an impacting chamber after the—R_(m) groups have been attached to graphene sheets and then adding thenew functionalizing agents to the impacting chamber and resuming theimpacting operation. The resulting graphene sheets or platelets includecompositions of the formula: [NGP]-A_(m), where A is selected from OY,NHY, O═C—OY, P═C—NR′Y, O═C—SY, O═C—Y, —CR′1-OY, N′Y or C′Y, and Y is anappropriate functional group of a protein, a peptide, an amino acid, anenzyme, an antibody, a nucleotide, an oligonucleotide, an antigen, or anenzyme substrate, enzyme inhibitor or the transition state analog of anenzyme substrate or is selected from R′—OH, R′—NR′₂, R′SH, R′CHO, R′CN,R′X, R′N⁺(R′)₃X⁻, R′SiR′₃, R′Si(—OR′—)_(y)R′_(3-y), R′Si(—O—SiR′₂—)OR′,R′—R″, R′—N—CO, (C₂H₄O—)_(w)H, (—C₃H₆O—)_(w)H, (—C₂H₄O)_(w)—R′,(C₃H₆O)_(w)—R′, R′, and w is an integer greater than one and less than200.

The NGPs may also be functionalized to produce compositions having theformula: [NGP]-[R′-A]_(m), where m, R′ and A are as defined above. Thecompositions of the invention also include NGPs upon which certaincyclic compounds are adsorbed. These include compositions of matter ofthe formula: [NGP]-[X—R_(a)]_(m), where a is zero or a number less than10, X is a polynuclear aromatic, polyheteronuclear aromatic ormetallopolyheteronuclear aromatic moiety and R is as defined above.Preferred cyclic compounds are planar. More preferred cyclic compoundsfor adsorption are porphyrins and phthalocyanines. The adsorbed cycliccompounds may be functionalized. Such compositions include compounds ofthe formula, [NGP]-[X-A_(a)]_(m), where m, a, X and A are as definedabove.

The functionalized NGPs of the instant invention can be prepared bysulfonation, electrophilic addition to deoxygenated platelet surfaces,or metallation. The graphitic platelets can be processed prior to beingcontacted with a functionalizing agent. Such processing may includedispersing the platelets in a solvent. In some instances the plateletsmay then be filtered and dried prior to contact. One particularly usefultype of functional group is the carboxylic acid moieties, whichnaturally exist on the surfaces of NGPs if they are prepared from theacid intercalation route discussed earlier. If carboxylic acidfunctionalization is needed, the NGPs may be subjected to chlorate,nitric acid, or ammonium persulfate oxidation.

Carboxylic acid functionalized graphitic platelets are particularlyuseful because they can serve as the starting point for preparing othertypes of functionalized NGPs. For example, alcohols or amides can beeasily linked to the acid to give stable esters or amides. If thealcohol or amine is part of a di- or poly-functional molecule, thenlinkage through the 0- or NH-leaves the other functionalities as pendantgroups. These reactions can be carried out using any of the methodsdeveloped for esterifying or aminating carboxylic acids with alcohols oramines as known in the art. Examples of these methods can be found in G.W. Anderson, et al., J. Amer. Chem. Soc. 96, 1839 (1965), which ishereby incorporated by reference in its entirety. Amino groups can beintroduced directly onto graphitic platelets by treating the plateletswith nitric acid and sulfuric acid to obtain nitrated platelets, thenchemically reducing the nitrated form with a reducing agent, such assodium dithionite, to obtain amino-functionalized platelets.

The following examples serve to provide the best modes of practice forthe present invention and should not be construed as limiting the scopeof the invention:

Example 1: Isolated NGP (Graphene Sheets) from Flake Graphite ViaPolypropylene Powder-Based Carrier

In an experiment, 1 kg of polypropylene pellets, 50 grams of flakegraphite, 50 mesh (average particle size 0.18 mm; Asbury Carbons, AsburyN.J.) and 250 grams of magnetic stainless steel pins (RaytechIndustries, Middletown Conn.) were placed in a ball mill container. Theball mill was operated at 300 rpm for 4 hours. The container lid wasremoved and stainless steel pins were removed via a magnet. The polymercarrier material was found to be coated with a dark carbon layer.Carrier material was placed over a 50 mesh sieve and a small amount ofunprocessed flake graphite was removed. Coated carrier material was thenplaced in a crucible in a vented furnace at 600° C. After cooling down,the furnace was opened to reveal a crucible full of isolated graphenesheet powder.

Although polypropylene (PP) is herein used as an example, the carriermaterial for making isolated graphene sheets is not limited to PP or anypolymer (thermoplastic, thermoset, rubber, etc.). The carrier materialcan be a glass, ceramic, metal, or other organic material, provided thecarrier material is hard enough to peel off graphene sheets from thegraphitic material (if the optional impacting balls are not present).

Example 2: NGP from Expanded Graphite Via ABS Polymer

In an experiment, 100 grams of ABS pellets, as solid carrier materialparticles, were placed in a 16 oz plastic container along with 5 gramsof expanded graphite. This container was placed in an acoustic mixingunit (Resodyn Acoustic mixer), and processed for 30 minutes. Afterprocessing, carrier material was found to be coated with a thin layer ofcarbon. Carrier material was placed in acetone and subjected toultrasound energy to speed dissolution of the ABS. The solution wasfiltered using an appropriate filter and washed four times withadditional acetone. Subsequent to washing, filtrate was dried in avacuum oven set at 60° C. for 2 hours.

Example 3: Functionalized Graphene from Meso-Carbon Micro Beads (MCMBs)Via PLA

In one example, 100 grams of PLA pellets (carrier material) and 2 gramsof MCMBs (China Steel Chemical Co., Taiwan) were placed in a vibratoryball mill, which also contains particles of magnetic stainless steelimpactor and processed for 2 hours. Subsequently, DETA was added and thematerial mixture was processed for an additional 2 hours. The vibratorymill was then opened and the carrier material was found to be coatedwith a dark coating of graphene. The magnetic steel particles wereremoved with a magnet. The carrier material was rinsed with isopropylalcohol and placed on a vacuum filter. The vacuum filter was heated to160° C. and vacuum was applied, resulting in removal of the PLA.

In separate experiments, the following functional group-containingspecies were introduced to the graphene sheets produced: an amino acid,sulfonate group (—SO₃H), 2-Azidoethanol, polyamide (caprolactam), andaldehydic group. In general, these functional groups were found toimpart significantly improved interfacial bonding between resultinggraphene sheets and epoxy, polyester, polyimide, and vinyl ester matrixmaterials to make stronger polymer matrix composites. The interfacialbonding strength was semi-quantitatively determined by using acombination of short beam shear test and fracture surface examinationvia scanning electronic microscopy (SEM). Non-functionalized graphenesheets tend to protrude out of the fractured surface without anyresidual matrix resin being attached to graphene sheet surfaces. Incontrast, the fractured surface of composite samples containingfunctionalized graphene sheets do not exhibit any bare graphene sheets;any what appears to be graphene sheets were completely embedded in aresin matrix.

Example 4: NGP from HOPG Via Glass Beads and SPEX Mill

In an experiment, 5 grams of glass beads were placed in a SPEX millsample holder (SPEX Sample Prep, Metuchen, N.J.) along with 0.25 gramsof HOPG derived from graphitized polyimide. This process was carried outin a 1% “dry room” to reduce the condensation of water onto thecompleted product. The SPEX mill was operated for 10 minutes. Afteroperation, the contents of the sample holder were transferred to a waterbath subjected to ultrasonication, which helps to separate graphenesheets from glass bead surfaces. The remaining material in the weightdish was a mixture of single layer graphene (86%) and few layergraphene.

Example 5: Production of Few Layer Graphene Via Wax-Based Carrier

In one example, 100 grams of hard machining wax in pellet form (F-14green, Machinablewax.com, Traverse City Mich., Hardness 55, Shore “D”)was mixed with 10 grams of vein graphite (40 mesh size, Asbury Carbons,Asbury N.J.) and loaded into a vibratory mill. The material wasprocessed for 4 hours, and the vibratory mill was opened. The waxpellets were found to be carbon coated. These pellets were removed fromthe mill, melted, and re-pelletized, resulting in 103 grams ofgraphene-loaded wax pellets. The wax pellets were placed again in thevibratory mill with an additional 10 grams of vein graphite, andprocessed for 4 hours. The resultant material was pelletized andprocessed with an additional 10 grams of vein graphite, creating awax/graphene composite with a graphene filler level of about 8.9%. Thewax carrier material was then dissolved in hexane and transferred intoacetone via repeated washing, then separated from acetone viafiltration, producing isolated, pristine NGP. The graphene plateletswere dried in a vacuum oven at 60° C. for 24 hours, and then surfacearea was measured via nitrogen adsorption BET.

Example 6: Low Temperature Metal Particles as the Carrier Material

In one example, 100 grams of tin (45 micron, 99.9% purity, GoodfellowInc; Coraopolis, Pa.) was mixed with 10 grams of vein graphite (40 meshsize, Asbury Carbons, Asbury N.J.) and loaded into a vibratory mill. Thematerial was processed for 2 hours, and the vibratory mill was opened.The tin powder was found to be carbon coated. These pellets were removedfrom the mill with tin being melted by heat and filtered using a vacuumfilter. The specific surface area of the resulting graphene material wasmeasured via nitrogen adsorption BET. A similar procedure was conductedusing zinc particles as the solid carrier material.

Example 7: Isolated NGP from Natural Graphite Particles Via Polyethylene(PE) Beads and Ceramic Impacting Balls

In an experiment, 0.5 kg of PE beads (as a solid carrier material), 50grams of natural graphite (source of graphene sheets) and 250 grams ofzirconia powder (impacting balls) were placed in containers of aplanetary ball mill. The ball mill was operated at 300 rpm for 4 hours.The container lid was removed and zirconia beads (different sizes andweights than graphene-coated PE beads) were removed through a vibratoryscreen. The polymer carrier material was found to be coated with a darkcarbon layer. Carrier material was placed over a 50 mesh sieve and asmall amount of unprocessed flake graphite was removed. Coated carriermaterial was then placed in a crucible in a vented furnace at 600° C.After cooling down, the furnace was opened to reveal a crucible full ofisolated graphene sheet powder (>95% single-layer graphene).

Comparative Example 1: NGP Via Hummer's Process

Graphite oxide as prepared by oxidation of graphite flakes with sulfuricacid, nitrate, and permanganate according to the method of Hummers [U.S.Pat. No. 2,798,878, Jul. 9, 1957]. Upon completion of the reaction, themixture was poured into deionized water and filtered. The graphite oxidewas repeatedly washed in a 5% solution of HCl to remove most of thesulphate ions. The sample was then washed repeatedly with deionizedwater until the pH of the filtrate was neutral. The slurry wasspray-dried and stored in a vacuum oven at 60° C. for 24 hours. Theinterlayer spacing of the resulting laminar graphite oxide wasdetermined by the Debey-Scherrer X-ray technique to be approximately0.73 nm (7.3 A). This material was subsequently transferred to a furnacepre-set at 650° C. for 4 minutes for exfoliation and heated in an inertatmosphere furnace at 1200° C. for 4 hours to create a low densitypowder comprised of few layer reduced graphene oxide (RGO). Surface areawas measured via nitrogen adsorption BET.

The RGO sheets were made into a disc of RGO paper 1 mm thick using avacuum-assisted filtration procedure. The electrical conductivity ofthis disc of RGO paper was measured using a 4-point probe technique. Theconductivity of this RGO disc was found to be approximately 550 S/cm. Incontrast, the graphene paper discs made from the pristine graphenesheets with the presently invented chemical-free process exhibits anelectrical conductivity in the range of 1,500 to 4,500 S/cm. Thedifferences are quite dramatic.

The invention claimed is:
 1. A method of producing isolated graphene sheets directly from a graphitic material, said method comprising: a) mixing multiple particles of a graphitic material having never been previously intercalated or chemically oxidized and multiple particles of a solid carrier material to form a mixture in an impacting chamber of an energy impacting apparatus; b) operating said energy impacting apparatus with a frequency and an intensity for a length of time sufficient for peeling off graphene sheets from said graphitic material and transferring said graphene sheets directly from said graphitic material to surfaces of said solid carrier material particles to produce graphene-coated solid carrier particles inside said impacting chamber; and c) separating said graphene sheets from said surfaces of said solid carrier material particles to produce said isolated graphene sheets, wherein the energy impacting apparatus is a vibratory ball mill, high energy mill, basket mill, agitator ball mill, continuous ball mill, stirred ball mill, pressurized ball mill, freezer mill, or vibratory sieve, wherein said separating includes a step of dissolving, melting, vaporizing, sublimating, or burning off said solid carrier material to separate said graphene sheet, wherein said impacting chamber further contains a functionalizing agent and said graphene sheets contain chemically functionalized graphene wherein said functionalizing agent contains O═C—SY, and Y is a functional group of a protein, a peptide, an enzyme, an antibody, a nucleotide, an oligonucleotide, an antigen, or an enzyme substrate, enzyme inhibitor or the transition state analog of an enzyme substrate or is selected from R′—OH, R′—NR′₂, R′SH, R′CHO, R′CN, R′X (a halide), R′N⁺(R′)₃X⁻, R′SiR′₃, R′Si(—OR′—)_(y)R′_(3-y), R′Si(—O—SiR′₂—)OR′, R′—R″, R′—N—CO, (C₂H₄O—)_(w)H, (—C₃H₆)—)_(w)H, (—C₂H₄O)_(w)—R′, (C₃H₆O)_(w)—R′, wherein R′ is selected from hydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, or poly(alkylether), and R″ is fluoroalkyl, fluoroaryl, fuorocycloalkyl, fluoroaralkyl, or cycloaryl, and w is an integer greater than one and less than 200, wherein y is an integer of less than
 3. 2. The method of claim 1, wherein a plurality of impacting balls or media are added to the impacting chamber of said energy impacting apparatus.
 3. The method of claim 1, wherein said impacting chamber of said energy impacting apparatus further contains a protective fluid.
 4. A method of producing isolated graphene sheets directly from a graphitic material, said method comprising: a) mixing multiple particles of a graphitic material having never been previously intercalated or chemically oxidized and multiple particles of a solid carrier material to form a mixture in an impacting chamber of an energy impacting apparatus, wherein said solid carrier material is selected from solid particles of an organic, polymeric, metal, or glass; b) operating said energy impacting apparatus with a frequency and an intensity for a length of time sufficient for peeling off graphene sheets from said graphitic material and transferring said graphene sheets directly from said graphitic material to surfaces of said solid carrier material particles to produce graphene-coated solid carrier particles inside said impacting chamber; and c) separating said graphene sheets from said surfaces of said solid carrier material particles to produce said isolated graphene sheets, wherein said separating includes a step of dissolving, melting, etching, vaporizing, sublimating, or burning off said solid carrier material to separate said graphene sheets, wherein the energy impacting apparatus is a vibratory ball mill, planetary ball mill, high energy mill, basket mill, agitator ball mill, continuous ball mill, stirred ball mill, pressurized ball mill, freezer mill, vibratory sieve, or resonant acoustic mixer wherein said impacting chamber further contains a functionalizing agent and said graphene sheets contain chemically functionalized graphene wherein said functionalizing agent contains O—C—SY, and Y is a functional group of a protein, a peptide, an enzyme, an antibody, a nucleotide, an oligonucleotide, an antigen, or an enzyme substrate, enzyme inhibitor or the transition state analog of an enzyme substrate or is selected from R′—OH, R′—NR′₂, R′SH, R′CHO, R′CN, R′X (a halide), R′N⁺(R′)₃X⁻, R′SiR′₃, R′Si(—OR′—)_(y)R′_(3-y), R′Si(—O—SiR′₂—)OR′, R′—R″, R′—N—CO, (C₂H₄O—)_(w)H, (—C₃H₆)—)_(w)H, (—C₂H₄O)_(w)—R′, (C₃H₆O)_(w)—R′, wherein R′ is selected from hydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, or poly(alkylether), and R″ is fluoroalkyl, fluoroaryl, fuorocycloalkyl, fluoroaralkyl, or cycloaryl, and w is an integer greater than one and less than 200, wherein y is an integer of less than
 3. 5. A method of producing isolated graphene sheets directly from a graphitic material, said method comprising: a) mixing multiple particles of a graphitic material having never been previously intercalated or chemically oxidized and multiple particles of a solid carrier material to form a mixture in an impacting chamber of an energy impacting apparatus, wherein said solid carrier material includes plastic beads, plastic pellets, wax pellets, polymer powder or polymer reactor spheres, glass beads or fibers, metal particles or wires, or a combination thereof; b) operating said energy impacting apparatus with a frequency and an intensity for a length of time sufficient for peeling off graphene sheets from said graphitic material and transferring said graphene sheets directly from said graphitic material to surfaces of said solid carrier material particles to produce graphene-coated solid carrier particles inside said impacting chamber; and c) separating said graphene sheets from said surfaces of said solid carrier material particles to produce said isolated graphene sheets, wherein the energy impacting apparatus is a vibratory ball mill, planetary ball mill, high energy mill, basket mill, agitator ball mill, continuous ball mill, stirred ball mill, pressurized ball mill, freezer mill, vibratory sieve, or resonant acoustic mixer, wherein said separating includes a step of dissolving, melting, etching, vaporizing, sublimating, or burning off said solid carrier material to separate said graphene sheets wherein said impacting chamber further contains a functionalizing agent and said graphene sheets contain chemically functionalized graphene wherein said functionalizing agent contains O═C—SY, and Y is a functional group of a protein, a peptide, an enzyme, an antibody, a nucleotide, an oligonucleotide, an antigen, or an enzyme substrate, enzyme inhibitor or the transition state analog of an enzyme substrate or is selected from R′—OH, R′—NR′₂, R′ SH, R′CHO, R′CN, R′X (a halide), R′N⁺(R′)₃X⁻, R′SiR′₃, R′Si(—OR′—)_(y)R′_(3-y), R′Si(—O—SiR′₂—)OR′, R′—R″, R′—N—CO, (C₂H₄O—)_(w)H, (—C₃H₆)—)_(w)H, (—C₂H₄O)_(w)—R′, (C₃H₆O)_(w)—R′, wherein R′ is selected from hydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, or poly(alkylether), and R″ is fluoroalkyl, fluoroaryl, fuorocycloalkyl, fluoroaralkyl, or cycloaryl, and w is an integer greater than one and less than 200, wherein y is an integer of less than
 3. 6. The method of claim 1, wherein said solid carrier material includes micron- or nanometer-scaled particles that can be dissolved in a solvent, melted above a melting temperature, etched away using an etching agent, vaporized or sublimated away, or burned off, and said method includes a step of dissolving, melting, etching, vaporizing, sublimating, or burning off said solid carrier material for separating said graphene sheets.
 7. The method of claim 1 wherein said graphitic material is selected from natural graphite, synthetic graphite, highly oriented pyrolytic graphite, graphite fiber, graphitic nano-fiber, graphite fluoride, oxidized graphite, chemically modified graphite, exfoliated graphite, recompressed exfoliated graphite, expanded graphite, meso-carbon micro-bead, or a combination thereof.
 8. The method of claim 1, wherein said graphitic material contains a non-intercalated and non-oxidized graphitic material that has never been previously exposed to a chemical or oxidation treatment prior to said mixing step.
 9. The method of claim 1 wherein said graphene sheets contain single-layer graphene sheets.
 10. The method of claim 1 wherein said graphene sheets contain at least 80% single-layer graphene or at least 80% few-layer graphene having no greater than 10 graphene layers.
 11. The method of claim 1 wherein said graphene sheets contain pristine graphene, oxidized graphene with less than 5% oxygen content by weight, graphene fluoride, graphene fluoride with less than 5% fluorine by weight, graphene with a carbon content no less than 95% by weight, or chemically modified graphene.
 12. The method of claim 1, wherein operating said energy impacting apparatus is conducted in a continuous manner using a continuous energy impacting device. 